In a hydraulic four-wheel-drive work vehicle including a variable displacement hydraulic pump unit, and first and second hydraulic motor units that are fluidly connected to the hydraulic pump unit and respectively and operatively drive first and second wheels positioned on one side and the other side in a vehicle longitudinal direction, in order to widen a speed-change range and/or compensate a difference in turning radius between the first and second wheels, it is useful to utilize a variable displacement hydraulic motor unit as at least one of the first and second hydraulic motor units.
For example, Patent Document 1 that is mentioned below discloses a hydraulic four-wheel-drive work vehicle including front and rear wheels that has a difference in turning radius between them, and front and rear hydraulic motor units that respectively drive the front and rear wheels, wherein one of the first and second hydraulic motor units is of a variable displacement type in order to compensate the difference in turning radius between the first and second wheels.
More specifically, the variable displacement hydraulic motor unit includes a hydraulic motor and a volume adjusting mechanism that varies a suction/discharge amount of the hydraulic motor. In the work vehicle disclosed in Patent Document 1, the volume adjusting mechanism is operated in accordance with a steering angle of the vehicle to vary the suction/discharge amount of the hydraulic motor accordingly, whereby driving speed of one of the front and rear wheels that are operatively driven by the variable displacement hydraulic motor can be increased or decreased in accordance with the difference in turning radius between the front and rear wheels.
As described above, the work vehicle disclosed in Patent Document 1 is configured so that the suction/discharge amount of the variable displacement hydraulic motor unit is varied in accordance with the steering angle of the vehicle, thereby making it possible to drive both the front and rear wheels while preventing slippage of the front and rear wheels when the vehicle makes a turn.
However, the variable displacement hydraulic motor unit disclosed in Patent Document 1 is configured so that the volume adjusting mechanism is mechanically operated. More specifically, a mechanical movement in accordance with the turning movement of the vehicle is transmitted to the volume adjusting mechanism, so that the suction/discharge amount of the hydraulic motor unit is defined in an unambiguous manner in accordance with the steering angle of the vehicle.
Therefore, a situation where the suction/discharge amount of the hydraulic motor unit becomes small at the time when vehicle starts to travel, which requires a large torque for travelling, may happen.